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A. Field of the Invention
The present invention relates to a method and device for selecting constellation points representing data transmitted in a data communication system. Although the invention may be applied to any data communication system, it is particularly suited for systems where constellation signal points are selected from non-uniformly spaced, single-dimensional signal space.
B. Description of the Related Art
A majority of today""s public telephone network transmits information digitally as a series of 8-bit bytes or octets at an 8 KHz clock rate. Portions of today""s telephone network, however, are not equipped for direct digital transmission. For example, the subscriber loop between the telephone company and the subscriber""s home is typically an analog channel. To transmit information over this analog channel, digital information is converted to an analog voltage amplitude known as pulse code modulation (xe2x80x9cPCMxe2x80x9d). The PCM signal is transmitted to the subscriber over the analog channel, and then translated back to the digital information.
This digital-to-analog (xe2x80x9cD/Axe2x80x9d) and analog-to-digital (xe2x80x9cA/Dxe2x80x9d) conversion of octet values to PCM voltage levels are typically performed in accordance with a non-linear quantizing rule. In North America, this conversion rule is known as xcexc-law. A similar non-linear sampling technique known as A-law is used in other geographic areas of the world such as Europe. The non-linear A/D and D/A conversion is generally performed by a codec (coder/decoder) device located at the interface between the digital network and the analog channel.
For high data transmission rates over the analog channel, it is desirable to use a large number of PCM levels to represent the transmitted data. Often, however, only a selected number of PCM levels, known as a constellation of signal points, is used due to the impairments of the transmission channel. For example, on noisy channels it is difficult to distinguish between PCM voltage levels that are spaced close together, resulting in data transmission errors. To distinguish between different PCM levels, the noise level may require an increased spacing between PCM levels. The spacing between selected PCM voltage levels can be increased by using a reduced subset of the available PCM levels. Using a limited number of PCM levels, however, limits the data rates that may be achieved on noisy channels.
In addition, the selection of PCM levels is also constrained by limits on the amount of power that can be transmitted onto the analog channel. The power of the transmitted PCM levels must be below this defined power level. Thus, higher power PCM levels that increase the spacing between PCM levels may not be selected. In this way, the power level limits the selection of constellation points and thus the noise performance and data transmission rate.
Accordingly, it is desirable to select signal constellations having PCM levels spaced at least a minimum distance apart as large as possible to improve noise performance, while staying within the allowable power level.
It is an object of the present invention to address the problem of selecting a constellation of PCM levels to enable the transmission of data at high transmission rates. In particular, it is an object of the present invention to select a constellation of points optimized to communicate at high data rates over analog channels which are subject to noise.
It is another object of the present invention, to select constellation points of PCM levels so that the minimum distance between PCM levels of the selected constellation are as great as possible while maintaining the power level of the constellation below a defined power limit.
It is a further object of the invention to select constellation points that reduce the number of PCM levels that are at the minimum distance spaced apart.
It is yet another object of the invention to select constellation points that provide the same number of points at lower power levels.
Described herein is a method and apparatus that improves the noise performance of modems transmitting data using a constellation of selected PCM levels. The described embodiment selects a constellation of points having a total number of PCM levels and an upper power limit that Improves the noise performance of the modem. The described embodiment selects PCM levels to increase the minimum spacing between PCM levels as great as possible while staying within the upper power limit for a constellation having a desired number of PCM levels. To minimize the power level of the selected constellation, constellation points are selected starting from PCM levels at linear values near zero to PCM levels at higher linear values. To improve noise performance, the number of occurrences of the minimum distance between PCM levels is also reduced without exceeding the upper power level. To reduce occurrences of the minimum distance between constellation points, PCM levels at the minimum distance from the previously selected PCM level may be skipped. The number of PCM levels spaced at the minimum distance is thus reduced to an optimum number without exceeding the upper power level. Software, firmware or hardware may be utilized for implementing the invention.